Sulfonated-perfluorocyclobutane polyelectrolyte membranes for fuel cells

ABSTRACT

A process for preparing a polymer comprising sulfonating a perfluorocyclobutane polymer with a sulfonating agent to form a sulfonated perfluorocyclobutane polymer, wherein the sulfonating agent comprises oleum or SO 3  is provided. A process for preparing proton exchange membranes and fuel cells comprising the proton exchange membrane are also provided.

BACKGROUND OF THE INVENTION

The present invention relates to electrochemical conversion cells,commonly referred to as fuel cells, which produce electrical energy byprocessing first and second reactants. For example, electrical energycan be generated in a fuel cell through the reduction (cathode reaction:O₂+4H⁺+4e⁻→2H₂O) of an oxygen-containing gas and the oxidation (anodereaction: 2H₂→4H⁺+4e⁻) of a hydrogenous gas. By way of illustration andnot limitation, a typical cell comprises a membrane electrode assemblypositioned between a pair of flow fields accommodating respective onesof the reactants. More specifically, a cathode flowfield plate and ananode flowfield plate can be positioned on opposite sides of themembrane electrode assembly. The voltage provided by a single cell unitis typically too small for useful automotive power application so it iscommon to arrange a plurality of cells in a conductively coupled “stack”to increase the electrical output of the electrochemical conversionassembly.

By way of background, the conversion assembly generally comprises amembrane electrode assembly, an anode flowfield, and a cathodeflowfield. The membrane electrode assembly in turn comprises a protonexchange membrane separating an anode and cathode. The membraneelectrode assembly generally comprises, among other things, a catalystsupported by a high surface area support material and is characterizedby enhanced proton conductivity under wet conditions. For the purpose ofdescribing the context of the present invention, it is noted that thegeneral configuration and operation of fuel cells and fuel cell stacksis beyond the scope of the present invention. Rather, the presentinvention is directed to particular polyelectrolyte membranes, processesfor preparing polyelectrolyte membranes and polyelectrolyte membranefuel cells. Regarding the general configuration and operation of fuelcells and fuel cell stacks, applicants refer to the vast collection ofteachings covering the manner in which fuel cell “stacks” and thevarious components of the stack are configured. For example, a pluralityof U.S. patents and published applications relate directly to fuel cellconfigurations and corresponding methods of operation. Morespecifically, FIGS. 1 and 2 of U.S. Patent Application Pub. No.2005/0058864, and the accompanying text, present a detailed illustrationof the components of a fuel cell stack—this particular subject matter isexpressly incorporated herein by reference.

BRIEF SUMMARY OF THE INVENTION

Recently, proton exchange or polyelectrolyte membrane (PEM) fuel cellshave attracted considerable interest as sources of non-polluting,high-density power for automotive propulsion. However, for widespreadcommercialization, low cost, high-performance PEMs with improveddurability are still being sought. Presently, PEM fuel cells operate attemperatures up to 95° C. with external humidification being required tomaintain proton conductivity that deteriorates rapidly as the membranesdry out. Perfluorosulfonic acid membranes have been the preferredmaterials for PEM, but they suffer from poor mechanical integrity andthey are expensive. Consequently, new alternative PEM materials arecontinuously being sought.

The present invention is directed to a process for preparing a polymer.The process comprises sulfonating a perfluorocyclobutane polymer with asulfonating agent to form a sulfonated perfluorocyclobutane polymer. Thesulfonating agent comprises oleum or SO₃.

In accordance with another embodiment of the present invention, aprocess for preparing a proton exchange membrane is provided. Theprocess comprises the steps of: (a) sulfonating a perfluorocyclobutanepolymer with a sulfonating agent to form a sulfonatedperfluorocyclobutane polymer and (b) forming the sulfonatedperfluorocyclobutane polymer into a proton exchange membrane. Thesulfonating agent comprises oleum or SO₃.

In accordance with yet another embodiment of the present invention, afuel cell is provided. The fuel cell comprises a proton exchangemembrane formed by sulfonating a perfluorocyclobutane polymer with oleumto form a sulfonated perfluorocyclobutane polymer and (b) forming thesulfonated perfluorocyclobutane polymer into a proton exchange membrane.The sulfonating agent comprises oleum or SO₃.

In accordance with a further embodiment of the present invention, aprocess for assembling a device comprises the act of preparing amembrane electrode assembly. The membrane electrode assembly compriseselectrically conductive material on either side of a proton exchangemembrane. The proton exchange membrane is prepared according to aprocess comprising the act of sulfonating a perfluorocyclobutane polymerwith a sulfonating agent to form a sulfonated perfluorocyclobutanepolymer, wherein the sulfonating agent comprises oleum or SO₃. Thedevice comprises an electrochemical conversion assembly comprising atleast one electrochemical conversion cell configured to convert firstand second reactants to electrical energy. The electrochemicalconversion cell comprises the membrane electrode assembly, an anodeflowfield portion and a cathode flowfield portion defined on oppositesides of the membrane electrode assembly. A first reactant supplyconfigured to provide a first reactant to an anode side of the membraneelectrode assembly via the anode flowfield portion, and a secondreactant supply configured to provide a second reactant to a cathodeside of the membrane electrode assembly via said cathode flowfieldportion.

BRIEF DESCRIPTION OF THE FIGURES

The following detailed description of specific embodiments of thepresent invention can be best understood when read in conjunction withthe following figures.

FIG. 1 is a graph depicting that the amount of sulfonation is determinedby the ratio of oleum to polymer used;

FIG. 2 is a graph depicting the conductivity vs. % relative humidity ofsulfonated perfluorocyclobutane-biphenyl vinyl ether (BPVE) polymers 4with different Ion Exchange Capacities (I.E.C.s);

FIG. 3 is a graph depicting conductivity vs. % relative humidity ofsulfonated perfluorocyclobutane-hexafluoroisopropylidene biphenyl vinylether (BPVE 6F) copolymers, 6 with different I.E.C.s;

FIG. 4 is a graph depicting water uptakes for various sulfonatedperfluorocyclobutane polymers;

FIG. 5 is a graph depicting volume swell at 25 and 100° C. in water forPFCB Polymers, plotted as semi-log (A) and linear graph (B);

FIG. 6 is a graph of fuel cell data of sulfonated BPVE polymersdepicting cell voltage (in volts) versus current density (inAmperes/cm²), which has been IR-corrected for the experimentallymeasured High Frequency Resistance (HFR); and

FIG. 7 is a graph of fuel cell data of sulfonated BPVE 6F copolymersdepicting cell voltage (in volts) versus current density (inAmperes/cm²), which has been IR-corrected for the experimentallymeasured High Frequency Resistance (HFR).

DETAILED DESCRIPTION

The inventors have discovered a new process for preparing new protonconducting membranes made with perfluorocyclobutanes polymers (PFCBs)having sulfonic acid groups (SPFCBs,), which may be used in PEM fuelcells that can operate over a broad range of relative humidity and attemperatures around 95° C. The properties of the SPFCB films aredependent on the chemical structure and the ion exchange capacity of thefilm, which can be tailored by the reaction conditions used. TheseSPFCB-films are reasonable alternatives to perfluorosulfonic acidmembranes, presently being used in PEM fuel cells, because thesulfonated polymers have high intrinsic proton conductivity and inherentdimensional-, hydrolytic- and high-temperature stability.

PFCBs are commercially available from Tetramer Technologies, underlicense agreements from Dow Chemical. Examples of PFCBs are providedwith the structures 1-3:

The synthesis of PFCBs is described in U.S. Pat. Nos. 5,037,917 and5,159,037—this particular subject matter is expressly incorporatedherein by reference.

To form potentially useful PEMs, subsequent sulfonation of the PFCBs isrequired. Prior art teaches a sulfonation procedure that useschlorsulfonic acid, which has limited synthesis utility and scope andwhich produces inconsistent membrane materials. The inventors havediscovered a novel process for synthesizing SPFCBs. The processcomprises sulfonating a perfluorocyclobutane polymer with a sulfonatingagent to form a sulfonated perfluorocyclobutane polymer. The sulfonatingagent comprises oleum, SO₃ or a combination thereof. One skilled in theart will appreciate that various PFCBs are available for use in thepresent process, any of which may be employed herein. In one embodiment,the PFCB comprises the formula:

wherein X is O or S;R is

n is greater than about 20.In one embodiment, n is from about 20 to about 500. Specific examples ofsuch PFCBs include, but are not limited to, structures 1-3, defined indetail above.

In addition, one skilled in the art will appreciate that variousconcentrations of the sulfonating agent may be employed to sulfonate aPFCB polymer, any of which may be employed herein. In one embodiment,the oleum comprises 10% oleum. In another embodiment, the oleumcomprises 20% oleum. In yet another embodiment, the oleum comprises 30%oleum. Moreover, one skilled in the art will appreciate that variousSPFCBs may be formed from reacting a PFCB with a sulfonating agent. Inone embodiment, the sulfonated polymers have between 0-2 sulfonic acidsper repeat unit. Examples of such SPFCBs include, but are not limitedto, structures 4-6:

Furthermore, one skilled in the art will appreciate the variousexperimental parameters in which the process for preparing thesulfonated polymer may be performed, any of which may be employedherein. In one embodiment, the process further comprises the step ofdissolving the PFCB polymer in methylene chloride prior to sulfonatingthe PFCB. In another embodiment, the process is performed from about−20° C. to about 200° C.

In accordance with another embodiment of the present invention, aprocess for preparing a proton exchange membrane is provided. Theprocess comprises the steps of: (a) sulfonating a perfluorocyclobutanepolymer with a sulfonating agent to form a sulfonatedperfluorocyclobutane polymer and (b) forming the sulfonatedperfluorocyclobutane polymer into a proton exchange membrane. Thesulfonating agent comprises oleum or SO₃.

In accordance with yet another embodiment of the present invention, afuel cell is provided. The fuel cell comprises a proton exchangemembrane formed by sulfonating a perfluorocyclobutane polymer with asulfonating agent to form a sulfonated perfluorocyclobutane polymer and(b) forming the sulfonated perfluorocyclobutane polymer into a protonexchange membrane. The sulfonating agent comprises oleum or SO₃. Asshown in the Example, SPFCB membranes prepared according to the presentinvention have markedly improved and consistent fuel cell performancecompared with those of the prior art. In one embodiment, the SPFCBcopolymer has an ion exchange capacity of from about 0.6 to about 2.5meq/gram. In another embodiment, the SPFCB copolymer has an ion exchangecapacity of from about 1.3 to about 2.0 meq/gram.

In accordance with a further embodiment of the present invention, aprocess for assembling a device comprises the act of preparing amembrane electrode assembly. The membrane electrode assembly compriseselectrically conductive material on either side of a proton exchangemembrane. The proton exchange membrane is prepared according to aprocess comprising the act of sulfonating a perfluorocyclobutane polymerwith a sulfonating agent to form a sulfonated perfluorocyclobutanepolymer, wherein the sulfonating agent comprises oleum or SO₃. Thedevice comprises an electrochemical conversion assembly comprising atleast one electrochemical conversion cell configured to convert firstand second reactants to electrical energy. The electrochemicalconversion cell comprises the membrane electrode assembly, an anodeflowfield portion and a cathode flowfield portion defined on oppositesides of the membrane electrode assembly. A first reactant supplyconfigured to provide a first reactant to an anode side of the membraneelectrode assembly via the anode flowfield portion, and a secondreactant supply configured to provide a second reactant to a cathodeside of the membrane electrode assembly via said cathode flowfieldportion.

EXAMPLE

Treatment of Polymers with Structures 1, 2, and 3 with 30% Oleum.

The properties of the SPFCB films are dependent on the chemicalstructure and the ion exchange capacity of the films, and performancecan be tailored by the reaction conditions used. The amount of oleumspecified in Table 1 is added to the respective polymer 1, 2 or 3, (ca 1gram) dissolved in methylene chloride (5 mL) in a screw cap jar. The jarlid is secured and then the jar is shaken vigorously. A purple gelimmediately forms, and then the jar is placed on a roll mill for between0.5 and 1 hour. A clear liquid phase separates, which is decanted offand discarded, and the purple solid is added to vigorously stirred water(250 mL) using a Waring blender. The polymer becomes swollen whitecrumbs, which are isolated by vacuum filtration, washed with water andthen air-dried. The sulfonated polymer (ca 1 g) readily dissolves intetrahydrofuran (4 mL) and methanol (2 mL).

The solution is filtered through an 80—micron polypropylene filter clothand is cast onto a glass plate (8-inch×8 inch) using a six-inch coatingapplicator bar with an 18-mil coating gap. The coated film is coveredwith a Pyrex glass dish, and then air-dried (to touch in about 10minutes). A free-standing film is produced after floating the film offthe glass with water. After extensive washing with water and air-drying,a 25-micrometer thick film is obtained. Pieces of the film (ca 0.02 g)are characterized by titration with standardized sodium hydroxide(0.010N) to determine its acid ion exchange capacity (see FIG. 1), bymeasuring proton conductivity versus percent relative humidity (seeFIGS. 2 and 3), and by determining water uptake after 1-hour immersionin water at 25 and 100° C. (boiling water), see FIG. 4. Percent volumeswell is determined for the PFCB polymers as well, see FIG. 5. A pieceof the film (4-inch×4-inch) is then tested in a fuel cell usingelectrodes made of catalyst coated diffusion media (CCDM) made with acoating of platinum on carbon (Tanaka) that is milled with A-K 900 SSPFSA ionomer solution and then coated on top of carbon-fiber diffusionmedia with a microporous layer. TABLE 1 Reaction Conditions to PrepareSulfonated Perfluorocyclobutane Polymers Made with 30% Oleum % H₂O Up-oleum/ IEC, meas IEC calcd take 100 C. polymer ratio FCD # Thickness μPolymer 4 BPVE 1.49 1.51 27.3 0.6171/1.0298 0.5992 BPVE 1.66 1.65 28.70.7244/1.0096 0.7175 BPVE 1.65 1.65 43.4 0.7289/1.0134 0.7193 1335 23BPVE 1.78 1.78 105 0.8168/1.0033 0.8141 1408 11 BPVE 1.92 1.90 1560.9655/1.0030 0.9626 1329 25 Polymer 6 BPVE 6F 1.52 1.51 950.6383/1.0030 0.6364 1410 22 BPVE 6F 1.58 1.56 73.8 0.6776/1.0280 0.65911303 BPVE 6F 1.73 1.73 164 0.7698/1.0412 0.7393 1313 BPVE 6F 1.80 1.80161 0.7927/1.0237 0.7743 1330 25 BPVE 6F 1.93 1.93 474 0.932/1.1160.8351 1320

The ion exchange capacity of the sulfonated polymers 4-6 is dependent onthe weight (or molar) ratio of the 30%-oleum used per gram of polymer(see FIG. 1). Thus, the I.E.C. of the respective polymers can bespecifically controlled by the amount of oleum added. The sulfonationreaction is quite fast and non-selective towards the different biphenylether groups in the PFCB backbone, and reaction times are less than onehour. The polymer 5 can be sulfonated with 30% oleum within 20 minutesunder the conditions described, but poor films were obtained with boththe starting polymer and the sulfonated polymer due to its low molecularweight. Thus, films of the sulfonated polymer 5 were too brittle to beevaluated.

Good fuel cell performances were obtained for films made of the SBPVEpolymer 4 with ion exchange capacities between 1.4 and 1.9 meq. ofsulfonic acid per gram (see FIG. 6). With the sulfonated copolymer 6,flooding of the fuel cell took place with the PEM that had an I.E.C. of1.9 meq/g of sulfonic acid. The other materials with between 1.52 and1.8 meq/g of sulfonic acid showed very good fuel cell performance underthe high-relative humidity conditions used (see FIG. 7).

All of the fuel cell results are summarized in Table 2. TABLE 2 Summaryof Fuel Cell Tests on PFCB Polymers. Fuel Water Volume Cell SulfonatingUptake Swell Polymer I.E.C. Test Agent V_(uncorr) LOW P, 50 kPa gV_(uncorr) High P, 170 kPa g Assessment 100 C. 100 C. BPVE- 1.4 FCDClSO₃H 0.361 v at 0.8 A/cm2 0.31 at 1.0 A/cm2 No Good x x 6F (6) 1295BPVE- 1.5 FCD 30% oleum 0.556 v at 1.2 A/cm2 0.529 v at 1.2 A/cm2 Good29 24 6F (6) 1410 BPVE- 1.58 FCD 30% oleum 0.604 v at 1.2 A/cm2 0.557 vat 1.2 A/cm2 Good 74 104 6F (6) 1303 BPVE 1.65 FCD 30% oleum 0.564 v at1.2 A/cm2 0.529 v at 1.2 A/cm2 Good 43 36 (4) 1335 BPVE 1.68 FCD ClSO₃H0.268 v at 0.4 A/cm2  0.27 v at 0.6 A/cm2 No Good x x (4) 1309 BPVE-1.73 FCD 30% oleum 0.600 at 1.2 A/cm2 0.586 v at 1.2 A/cm2 Good 164 2446F (6) 1313 BPVE- 1.75 FCD ClSO₃H 0.434 v at 1.2 A/cm2  0.57 v at 1.2A/cm2 Fair 36 x 6F (6) 1394 BPVE 1.78 FCD 30% oleum 0.629 v at 1.2 A/cm20.624 v at 1.2 A/cm2 Good 105 84 (4) 1408 BPVE- 1.8 FCD 30% oleum 0.625v at 1.2 A/cm2  0.63 v at 1.2 A/cm2 Good 161 151 6F (6) 1330 BPVE 1.9FCD 30% oleum 0.606 v at 1.2 A/cm2 0.637 v at 1.2 A/cm2 Good 156 238 (4)1329 BPVE- 1.93 FCD 30% oleum 0.311 v at 1.2 A/cm2 0.493 v at 1.2 A/cm2No Good 474 393 6F (6) 1320

In general, there is considerable concern about the oxidative stabilityof hydrocarbon membranes in fuel cells, and one ex-situ test consists ofan assessment of membrane stability on immersion in Fenton's reagent.The polymers 4 and 6 showed a 17 to 18 wt. % loss after 19 hours in aFenton's test solution made with 4-ppm Fe²⁺ (ferrous chloridetetrahydrate) and 3% hydrogen peroxide in an oven set at 70° C. Thiscompares with Nafion 112, which loses less than 0.5 wt. % under the sameconditions.

1. A process for preparing a polymer comprising sulfonating aperfluorocyclobutane polymer with a sulfonating agent to form asulfonated perfluorocyclobutane polymer, wherein the sulfonating agentcomprises oleum or SO₃.
 2. The process of claim 1, wherein theperfluorocyclobutane polymer comprises the formula:

wherein: X is O or S; R is

n is greater than about
 20. 3. The process of claim 2, wherein n is fromabout 20 to about
 50. 4. The process of claim 2, wherein theperfluorocyclobutane polymer comprises the formula:

wherein n is from about 20 to about
 500. 5. The process of claim 1,wherein the sulfonated perfluorocyclobutane polymer comprises 0-2sulfonic acid groups per repeating unit.
 6. The process of claim 1,wherein oleum comprises 10% oleum, 20% oleum or 30% oleum.
 7. Theprocess of claim 1, further comprising the step of dissolving theperfluorocyclobutane polymer in methylene chloride prior to sulfonatingthe polymer.
 8. The process of claim 1, wherein sulfonating aperfluorocyclobutane polymer yields a sulfonated perfluorocyclobutanepolymer with an ion exchange capacity of from about 0.6 to about 2.5meq/gram.
 9. The process of claim 8, wherein sulfonating aperfluorocyclobutane polymer yields a sulfonated perfluorocyclobutanepolymer with an ion exchange capacity of from about 1.3 to about 2.0meq/gram.
 10. The process of claim 1, wherein the process is performedfrom about −20° C. to about 200° C.
 11. A process for preparing a protonexchange membrane comprising the steps of: (a) sulfonating aperfluorocyclobutane polymer with a sulfonating agent to form asulfonated perfluorocyclobutane polymer and (b) forming the sulfonatedperfluorocyclobutane polymer into a proton exchange membrane, whereinthe sulfonating agent comprises oleum or SO₃.
 12. A fuel cell comprisingthe proton exchange membrane of claim
 11. 13. A process for assembling adevice, the process comprising the act of preparing a membrane electrodeassembly, wherein: the membrane electrode assembly compriseselectrically conductive material on either side of a proton exchangemembrane; wherein the proton exchange membrane is prepared according toa process comprising the act of sulfonating a perfluorocyclobutanepolymer with a sulfonating agent to form a sulfonatedperfluorocyclobutane polymer, wherein the sulfonating agent comprisesoleum or SO₃; and wherein the device comprises an electrochemicalconversion assembly comprising at least one electrochemical conversioncell configured to convert first and second reactants to electricalenergy, the electrochemical conversion cell comprising the membraneelectrode assembly, an anode flowfield portion and a cathode flowfieldportion defined on opposite sides of the membrane electrode assembly, afirst reactant supply configured to provide a first reactant to an anodeside of the membrane electrode assembly via the anode flowfield portion,and a second reactant supply configured to provide a second reactant toa cathode side of the membrane electrode assembly via said cathodeflowfield portion.